1. Field of the Invention
The present invention relates to an apparatus for detecting a state where no gear is engaged (simply referred to as a neutral state hereinafter) of a transmission gear, in an engine system including the transmission gear for transmitting an output torque of a vehicle mounted internal combustion engine to a later stage.
2. Description of Background Information
Transmission gear of a vehicle is provided for transmitting an output torque of a vehicle mounted internal combustion engine to wheels of the vehicle. The internal combustion engine and the transmission gear, in combination, can be designated as an engine system which operates as a source of a desired torque. Various control systems such as an air/fuel ratio control system are known as means for controlling an operation of such an engine system. In those control systems, it is necessary to detect various operational parameters such as rotational speed of engine in the engine system, throttle opening angle, other various parameters of engine operation, sift position of the transmission gear, and vehicle speed. Thus, a plurality of sensors for sensing the operational parameters are provided in those control systems.
Among those sensors, there is a so-called neutral switch which senses a neutral state of the transmission gear. As an example, Japanese Patent Publication No. 49-45256 discloses such a neutral switch.
The neutral switch such as disclosed in the above mentioned patent is generally a mechanical switch interconnected with a gear shift linkage of the transmission gear. Therefore, such a neutral switch is subjected to failures including not only a fault of the switch itself, but other troubles such as a short-circuit between lead wires. In the event that such a failure of the neutral switch occurs, a false signal indicating the neutral state may be supplied to the engine system, to hinder the proper control operation.